Precision machine vision inspection systems (or “vision systems” for short) can be utilized to obtain precise dimensional measurements of inspected objects and to inspect various other object characteristics. Such systems may include a computer, a camera and optical system, and a precision stage that is movable in multiple directions to allow workpiece inspection. One exemplary prior art system, that can be characterized as a general-purpose “off-line” precision vision system, is the commercially available QUICK VISION® series of PC-based vision systems and QVPAK® software available from Mitutoyo America Corporation (MAC), located in Aurora, Ill. The features and operation of the QUICK VISION® series of vision systems and the QVPAK® software are generally described, for example, in the QVPAK 3D CNC Vision Measuring Machine User's Guide, published January 2003, and the QVPAK 3D CNC Vision Measuring Machine Operation Guide, published September 1996, each of which is hereby incorporated by reference in their entirety. This type of system is able to use a microscope-type optical system and move the stage so as to provide inspection images either small or relatively large workpieces at various magnifications.
General purpose precision machine vision inspection systems, such as the QUICK VISION™ system, are also generally programmable to provide automated video inspection. Such systems typically include GUI features and predefined image analysis “video tools” such that operation and programming can be performed by “non-expert” operators. For example, U.S. Pat. No. 6,542,180 (hereinafter “the '180 patent”), which is incorporated herein by reference in its entirety, teaches a vision system that uses automated video inspection.
As taught in the '180 patent, automated video inspection metrology instruments generally have a programming capability that allows an automatic inspection event sequence to be defined by the user for each particular workpiece configuration. Such programming can be implemented as text-based programming, or through a recording mode that progressively “learns” the inspection event sequence by storing a sequence of machine control instructions corresponding to a sequence of inspection operations performed by a user, or through a combination of both methods. Such a recording mode is often referred to as “learn mode” or “training mode.” In either technique, the machine control instructions are generally stored as a part program that is specific to the particular workpiece configuration, and automatically performs a predetermined sequence of inspection operations during a “run mode” of operation.
In general, during a known sequence of autofocus operations the camera moves through a range of positions along a Z-axis and captures an image at each position. For each captured image, a focus metric is calculated and related to the corresponding position of the camera along the Z-axis at the time that the image was captured. One known method of autofocusing is discussed in “Robust Autofocusing in Microscopy,” by Jan-Mark Geusebroek and Arnold Smeulders in ISIS Technical Report Series, Vol. 17, November 2000, which is incorporated herein by reference, in its entirety. In order to determine a Z-axis position of the camera that corresponds to an autofocus image, the discussed method estimates a position of the camera along a Z-axis based on a measured amount of time during which the camera moves from a known original position on the Z-axis at a constant velocity along the Z-axis, until the image is acquired. During the constant velocity motion, the autofocus images are captured at 40 ms intervals (video rate). The disclosed method teaches that the video hardware captures frames at a fixed rate, and that the sampling density of the focusing curve can be influenced by adjusting the stage velocity. Another known autofocus method and apparatus is described in U.S. Pat. No. 5,790,710 (hereinafter “the '710 patent”), which is incorporated herein by reference, in its entirety. In the '710 patent a piezoelectric positioner is utilized in conjunction with a conventional motor-driven motion control system to control the Z-axis position motion while acquiring autofocus images. Another improved autofocus system and method is described in U.S. Pat. No. 7,030,351, which is commonly assigned and hereby incorporated by reference, in its entirety. In all these cases, a relatively large number of images are acquired during run mode as a basis for autofocusing prior to acquiring inspection images. Due to increasing computation speeds, computation speeds are becoming less relevant to inspection throughput, while the physical movements required for the systems and methods referred to above generally remain as a primary factor limiting inspection throughput. An autofocus system and method that can further improve throughput is desirable.
The present invention is directed to a system and method for providing inspection images at an improved rate. More specifically, a system and method are provided for quickly adjusting to acceptable approximate focus positions using a limited amount of physical movement.